Method for enhancing oxide to nitride selectivity through the use of independent heat control

ABSTRACT

A process for controlling the etch of a silicon dioxide layer at a high etch rate and high selectivity with respect to silicon nitride, particularly in a multilayer structure, by maintaining various portions of the etch chamber at elevated temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation application of co-pending U.S. applicationSer. No. 08/905,891, filed Aug. 4, 1997; which was a continuation ofU.S. application Ser. No. 08/152,755, filed Nov. 15, 1993, which issuedas U.S. Pat. No. 5,880,036 on Mar. 9, 1999; which was acontinuation-in-part of application Ser. No. 07/898,505, filed Jun. 15,1992, which issued as U.S. Pat. No. 5,286,344 on Feb. 15, 1994.

FIELD OF THE INVENTION

[0002] This invention relates to semiconductor manufacturing, and moreparticularly to a process for selectively etching a silicon dioxidelayer disposed on a silicon nitride layer, useful when etching featureshave submicron geometries.

BACKGROUND OF THE INVENTION

[0003] With geometries shrinking, it is becoming more difficult to alignsmall contacts in between closely spaced wordlines or other conductivestructures. Therefore, an etch is needed which would etch an oxide layerand stop on the underlying nitride layer. The highly selective etchshould also display consistency for manufacturing purposes.

[0004] Current manufacturing processes of multilayer structurestypically involve patterned etching of areas of the semiconductorsurface which are not covered by a pattern of protective photoresistmaterial. These etching techniques use liquid or wet etching materials,or dry etching with halogens or halogen-containing compounds.

[0005] Etching of the multilayer structures can also be conducted in agas phase using known techniques, such as plasma etching, ion beametching, and reactive ion etching. The use of gas plasma technologyprovides substantially anisotropic etching using gaseous ions, typicallygenerated by a radio frequency (RF) discharge

[0006] In gas plasma etching the requisite portion of the surface to beetched is removed by a chemical reaction between the gaseous ions andthe subject surface. In the anisotropic process, etching takes placeprimarily in the vertical direction so that feature widths substantiallymatch the photoresist pattern widths. Anisotropic etching is utilizedwhen feature sizing after etching must be maintained within specificlimits so as not to violate alignment tolerances or design rules.

[0007] Higher density multilayer structures such as 64 and 256 MegabitDRAM require an additional amount of alignment tolerance which can notbe addressed by current photolithographic means. In such applications,an etch stop technology could be used to supply the desired tolerance.

[0008] In an etch “stop” system, an etch “stop” layer is deposited onunderlying structures. The superjacent layer is disposed over theunderlying etch “stop” layer through which the desired patterns will bedefined. The etch “stop” layer will then be used to terminate the etchprocess once the superjacent layer has been completely removed in thedesired pattern locations. Thus, the etch “stop” layer acts to protectstructures underlying the etch “stop” layer from damage due to the drychemical etch of the superjacent layer.

[0009] The preferred etch “stop” material is silicon nitride because itsproperties are well known, and it is currently used for semiconductorfabrication. The preferred superjacent layer is silicon dioxide, orother oxide such as, BPSG.

[0010] The etch stop process must have three basic properties, namely,(1) a high etch rate for the superjacent layer which (2) producessubstantially vertical sidewalls, and (3) has a high selectivity of thesuperjacent layer being etched down to the etch “stop” layer.

[0011] A problem of profile control occurs with respect to etching of amultilayer structure having a silicon dioxide layer disposed on anunderlying silicon nitride layer. Profile control using pure chemicaletching (e.g., using hydrofluoric acid) tends to produce structures thatdo not have vertical sidewalls.

[0012] Dry etch processing usually produces a more vertical profilebecause of the ion bombardment aspect of the process. however, the dryetch process can produce a contact wall that slopes out from the bottom,rather than at an angle of 90°, if the wrong mix of process parametersare used. These parameters can include, but are not limited to;fluorocarbon, RF power, and pressure.

[0013] The same ion bombardment aspect of the dry etch process used toproduce straight sidewalls has a very negative effect on oxide tonitride selectivity. High energy ions needed to etch both oxide andnitride do so by disassociating a chemical bond at the oxide and/ornitride surface. However, the disassociation energy needed for nitrideis less than that required for oxide.

[0014] Hence, CH₂F₂ is added to offset the disassociation properties ofnitride as compared to oxide. The CH₂F₂ produces a polymer deposition anthe nitride surface that acts to passivate the nitride surface andthereby reduce the dry etch removal rate. However, the silicon dioxideetch rate is sustained at a much higher rate than that of siliconnitride.

[0015] Current etch process technology for etching an SiO₂ layer on anunderlying Si₃N₄ layer using a dry etcher, such as an RIE or MRIEetcher, cannot produce SiO₂-to-Si₃N₄ selectivities above 5-6:1 withadequate profile and SiO₂ etch rate characteristics.

[0016] Almost all of the current etch processes which involve highselective etches, rely on cooler temperatures to obtain thoseselectivities. See, for example, “Temperature Dependence of SiliconNitride Etching by Atomic Fluorine,” and “Selective Etching of SiliconNitride Using Remote Plasmas of CF₄ and SF₆,” both by Lee M.Loewenstein. The latter reference uses an Arrhenius plot having anegative slope to illustrate that the nitride etch rate increases as afunction of substrate temperature.

[0017] Therefore, a need exists for a process of etching a SiO₂ layer onan underlying Si₃N₄ layer, at a high SiO₂ etch rate. Furthermore, thereexists a need for an etch at a high selectivity of SiO₂ with respect tothe underlying Si₃N₄, to form an etched multilayer structure at acontrolled predetermined profile in which the resulting sidewalls aresubstantially normal to the substrate.

SUMMARY OF THE INVENTION

[0018] The present invention provides unexpected and very keyimprovements over the current etch processes. The present inventionteaches away from current thought, by using increased temperatures toachieve increased selectivity. In addition to improved selectivity, thehigher temperatures help reduce the polymer build-up inside the chamber.

[0019] The process of the present invention meets the above-describedexisting needs by forming an etched multilayer structure, in which thesidewalls of the SiO₂ layer are substantially normal to the substrate,at a high SiO₂ etch rate, and at a high selectivity of SiO₂ with respectto the underlying Si₃N₄. This is accomplished by heating variousportions of the etch chamber while employing a process for etching theSiO₂ layer down to the Si₃N₄ stop layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will be better understood from reading thefollowing description of nonlimitative embodiments, with reference tothe attached drawings, wherein below:

[0021]FIG. 1 is a schematic cross-section of a multilayer structurehaving a silicon dioxide layer disposed on a silicon nitride “etch” stoplayer, prior to etching with the fluorinated chemical etchant system ofthe present invention;

[0022]FIG. 2 is a schematic cross-section of the multilayer structure ofFIG. 1, after the etch step according to the process of the presentinvention;

[0023]FIG. 3 is a plot of oxide:nitride selectivity in relation to boththe silicon anode temperature and the addition of CH₂F₂; and

[0024]FIGS. 4a-4 c are Arrhenius plots illustrating the unexpectedresults obtained with the process of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0025] The inventive process herein is directed towards anisotropicallyetching a multilayer structure comprising a silicon dioxide outer layeron an underlying silicon nitride “stop” layer.

[0026] Referring to FIG. 1, a multilayer structure, which is formed byconventional techniques, is depicted. It will serve as a representativeexample. The multilayer structure of FIG. 1, generally designated as 10,is shown prior to etching.

[0027] The multilayer structure 10 comprises a plurality of structurallayers which are sequentially disposed on an underlying siliconstructure or wafer 18. Multilayer structure 10 comprises a plurality ofstructural layers including a layer 14 having a major outer surface 14a. Structural layer 14 is fabricated of SiO₂.

[0028] Generally, an undoped oxide 15, referred to as a field oxide orgate oxide, is usually grown in a furnace. Doped oxide includes BPSG,PSG, etc. which are generally deposited on the silicon wafer with adopant gas (es) during a deposition process.

[0029] The outer structural layer 14 is deposited onto an adjacentintermediate structural layer 16. Layer 16 includes sidewalls and isfabricated of an etch “stop” layer of silicon nitride.

[0030] Also shown in FIG. 1, is a chemical etchant protective patternedlayer 12 which comprises a photoresist layer having a predeterminedarrangement of openings 12 a for forming a predetermined pattern inmultilayer structure 10. Typically, this is accomplished using asemiconductor photomask, and known conventional etch mask patterningtechniques.

[0031] The etch “stop” layer 16 is disposed on the field oxide 15,silicon substrate 18, and onto a plurality of polysilicon lines 17located adjacent to their respective sidewalls spacer elements 19.

[0032] As seen in FIG. 2, the preferred manner of etching of thestructural SiO₂ layer 14 down to etch “stop” layer 16 is by plasma etch.The gas plasma etch technique employed herein typically has an etchingarea in a plasma and-is generated under vacuum within the confines of anRF discharge unit.

[0033] The preferred plasma etch technique employed herein may includethe use of ECR (Electron Cyclotron Resonance), RIE, MIE, MERIE, PEreactive ion, point plasma etching, magnetically confined helicon andhelical resonator, PE, or magnetron PE. In plasma dry etchers, typicallythe upper electrode is powered while the lower electrode is grounded.

[0034] In RIE (Reactive Ion Etchers), the lower electrode is poweredwhile the upper electrode is grounded. In triode dry etchers, the upperand lower electrodes can be powered as well as the sidewall. In MERIE(magnetically enhanced reactive ion etch) magnets are used to increasethe ion density of the plasma. In ECR (Electron Cyclotron Resonance),the plasma is generated upstream from the main reaction chamber. Thisproduces a low ion energy to reduce damage to the wafer.

[0035] A semiconductor device is disposed in the desired etcher, withinan etching area, and is etched with a fluorinated chemical etchantsystem to form a predetermined pattern therein. The fluorinated chemicaletchant system comprises a chemical etchant composition, such as, forexample, CHF₃-CF₄Ar, and a CH₂F₂ additive material. The fluorinatedchemical etchant system is in a substantially gas phase during theetching of the multilayer structure 10.

[0036] The exposed SiO₂ layer 14 is selectively etched at a relativelyhigh etch rate down to the Si₃N₄ etch “stop” layer 16 by removingpredetermined portions of the SiO₂ layer 14 by chemically enhanced ionicbombardment. Some areas of the wafer still had SiO₂ available foretching, while other areas of the wafer had already reached the nitridelayer 16 where the etching process effectively stops because of polymerformation on the nitride surface. In this way, the etching process canprovide for the formation of sidewalls in etched layers which have asubstantially vertical profile.

[0037] The etching system employed in developing the process of thisinvention was the Applied Materials Precision 5000, a single waferplasma etching apparatus manufactured by Applied Materials of SantaClara, Calif. This apparatus comprises a mobile, double cassetteplatform, a transport chamber with an 8-wafer storage elevator, and from1-4 plasma etching chambers.

[0038] The mobile cassette platform is maintained at atmosphericpressure during the entire operation of the apparatus. It holds twocassettes of wafers, each capable of holding up to 25 wafers. Theplatform can be raised or lowered and moved laterally so that anyparticular wafer may be aligned with a narrow door located between theplatform and the transport chamber.

[0039] Nitrogen gas is fed through a flow control valve into thetransport chamber to vent the chamber to atmosphere. A robot transferarm in the transport chamber transfers wafers from the cassette on themobile cassette platform to the storage elevator in the transportchamber.

[0040] The transport chamber is connected to a two stage evacuation pumpwhich is used to evacuate the transport chamber and maintain it at asuitable pressure for transporting wafers from the elevator to theplasma etching chamber. This pressure was maintained at 75-125 mTorr.

[0041] The plasma etching chamber is connected to a turbo pump and thetwo stage pump which evacuates the chamber to a lower pressure than thatof the transport chamber. This pressure is typically less than 10 mTorr.

[0042] When the transport chamber and the plasma etching chamber havereached suitable pressures for wafer transfer, the robot arm transfers awafer from the wafer storage elevator to the plasma etch chamber.

[0043] The plasma etching chamber contains an upper, electricallygrounded electrode which also serves as the chamber sidewalls, and alower, RF powered electrode upon which the wafer is clamped during theplasma etch process, and a set of electromagnetic coils placed aroundthe chamber sidewalls.

[0044] In one embodiment of the present invention, an etch chamberhaving an upper electrode (or anode) which is comprised of silicon isused. It is believed that the silicon scavenges the free fluorine fromthe reaction, and thereby substantially prevents the free fluorine frometching the nitride layer 16.

[0045] In the process of the present invention, it has been unexpectedlyfound that an enhanced selectivity effect results with the addition ofCH₂F₂ while using a hot silicon plate (or anode). For CH₂F₂ flows above8 sccm, and silicon plate temperatures above 75° C., a significantresult occurs, as depicted in plot of FIG. 3. At a silicon platetemperature of 225° C., the selectivity of oxide:nitride increases from1.7:1 without CH₂F₂, to over 33:1 with the addition of 10 sccm

[0046] The chamber also contains a gas distribution plate connected tothe lid of the chamber, through which suitable feed gas mixtures are fedinto the chamber from a connected gas supply manifold.

[0047] When RF energy is applied to the lower electrode, the gas fedinto the chamber, via the gas distribution plate, is converted toplasma. The plasma contains reactive chemical species which etchselected unmasked portions of the wafer, which wafer is clamped to thelower electrode.

[0048] Electric power is applied to the electromagnetic coils whichsurround the chamber sidewalls. The magnetic field generated by thecoils increases the density of the plasma near the wafer surface. Athrottle valve located between the plasma etching chamber regulates thepressure of the chamber to processing values, generally in the range of10-350 mTorr.

[0049] The lower electrode is connected to a wafer cooling systemdesigned to maintain the wafer at a constant temperature during theplasma etch process. This system consists of two parts. The first is anapparatus providing a temperature controlled fluid which circulatesthrough a tunnel in the lower electrode.

[0050] The second part is an apparatus providing a pressure and flowcontrolled inert gas (typically helium) of-high thermal conductivitywhich is fed to the underside of wafer during etch via a channel throughthe lower electrode, opening to grooves on the top face of the lowerelectrode. The helium gas is contained behind the wafer by an O-ringseal which lies partially in a circular groove in the lower electrode.

[0051] The second part is referred to as a helium backside coolingsystem. During plasma etches, power is dissipated in the plasma throughthe ionization of the gaseous species. During the ionization process, alarge amount of heat is generated. The helium backside cooling systemallows the heat which has been imparted to the wafer, to be moreeffectively coupled to the temperature controlled lower electrode. Asthe pressure in the helium cooling system is increased, the wafertemperature more closely matches the temperature of the lower electrodethroughout the plasma process. Hence, a more stable and predictableprocess is possible.

[0052] When the clamp is lowered to clamp the wafer against the lowerelectrode, the wafer underside is held tightly against the O-ring seal.The seal prohibits leakage of the inert gas from underneath the wafer tothe plasma etch cavity.

[0053] The machine is governed by a programmable computer that isprogrammed to prompt the machine to evacuate and vent the transportchamber and plasma etching chamber, transfer wafers to and from thecassettes, elevator, and etch chamber, control the delivery of processgas, RF power, and magnetic field to the plasma etching chamber, andmaintain the temperature of the wafer in the plasma etching chamber, allat appropriate times and in appropriate sequence.

[0054] The multilayer 10 structure is then placed within the plasmaetching chamber, and etched with a fluorinated chemical etchant systemto form a predetermined pattern therein. The fluorinated chemicaletchant system of the present invention comprises a chemical etchantcomposition, such as CHF₃, CF₄, and Ar, and an additive material. Thefluorinated chemical etchant system is in a substantially gaseous phaseduring the etching of the multilayer structure 10.

[0055] In the case of the chemical etchant composition including CHF₃,CF₄ and Ar, and an additive material comprising CH₂F₂, the exposed SiO₂layer 14 is selectively etched at a relatively high etch rate and highselectivity down to the Si₃N₄ etch “stop” layer 16. Predeterminedportions of the SiO₂ layer 14 are removed using chemically enhancedionic bombardment of the gas phase etchant material.

[0056] An inert gas, preferably argon (Ar), is added to the etch plasma,as it tends to further enhance the uniformity of the etch process. Argonis preferred because of its weight and commercial availability, but theother inert gases can also be used.

[0057] Heating the chamber sidewall and electrode (i.e., the waferchuck) to higher than normal operating range, according to the presentinvention, produces an increase in oxide to nitride selectivity,contrary to the current teaching on high selectivity etching.

[0058] Heating the separate individual components of the chamber (e.g.,sidewalls, chuck, helium backside, etc.) produced varying degrees ofpositive results when there was an overall increase in temperature.

[0059] Current process temperatures for highly selective etches includemaintaining the etch chamber sidewalls at approximately 50° C., and thelower electrode at approximately 20° C. or below, and a helium backsidepressure in the approximate range of 4.0-12.0 torr.

[0060] In contrast, the preferred embodiment of the present inventioninvolves increasing the temperature of the chamber sidewalls to atemperature in the approximate range of 50° C.-100° C., and thetemperature of the lower electrode is in the approximate range of 30°C.-100° C., and preferably in the range of 30° C.-70° C. The heliumbackside cooling apparatus is maintained at a pressure in theapproximate range of 4.0 torr or less. Decreasing the pressure of thehelium backside cooling apparatus, essentially translates to increasingthe temperature.

[0061] The reaction chamber can be heated via a fluid system, in which afluid, such as, for example, water, at a desired temperature is flowedaround the chamber walls. Alternatively, a gas can be flowed to heat thesystem.

[0062] However, temperature increases in the lid or anode has producednegative results. Therefore, it is critical that the right combinationof higher temperatures be maintained to produce the best selectivity. Ifthe anode is increased to a temperature over 90° C., the photoresistlayer 12 will begin to burn and reticulate. This upper temperaturelimitation is governed by the masking material and should not be viewedas a hard limit.

[0063] It is believed that increasing the temperature, also increasesthe rate of generation of the particular polymer species, andconsequently is responsible for the increase in oxide to nitrideselectivity. By increasing the temperature of the chamber, chuck, andsidewall, the selectivity is increased. Further, as the backside heliumcooling was reduced, (in effect heating the wafer), the selectivity alsoincreased.

[0064] The use of temperature control in the present invention furtherhelps to minimize polymer build-up on the surfaces of the reactionchamber. Limiting polymer build-up substantially decreases possiblecontaminants, as well as downtime of the apparatus for cleaning.

[0065] Representative etch parameters were employed in the process foretching a multilayer structure 10 of the present invention. One havingordinary skill in the art will realize that the above values will varydepending on the make and model of the etcher used in the process.

[0066] The flow rates of the component gases, based on the total gasflow of the fluorinated chemical etchant system, used herein was asfollows: an etchant comprised of 16% CF4, 60% Ar, 9% CH2F2, and 13%CHF3, at a total pressure in the system of 225 mTorr, magnetic fieldmaintained at 75 gauss, and RF power applied at 425 watts.

[0067] The parameters of the present invention are within the followingapproximate ranges: an etchant material comprised of 14 sccm CH2F2, 25sccm CF4, 90 sccm AR, and 20 sccm CHF3, at a total pressure in thesystem of 225 mTorr magnetic field maintained at 75 gauss, and RF powerapplied at 425 watts.

[0068] Silicon dioxide and silicon nitride layers, 14 and 16respectively, were patterned with etch masks 12 having the appropriateetch mask openings 12 a and geometries. The wafers were then etched,thereby creating a substantially vertical profile in the respectivefilms.

[0069]FIGS. 4a, 4 b, and 4 c illustrate the etch selectivities whichwere obtained using the process of the present invention. FIG. 4adepicts the positive slope obtained on an Arrhenius plot, which slopeindicates that the nitride etch rate decreases as a function ofincreased electrode temperature.

[0070] All of the U.S. Patents cited herein are hereby incorporated byreference herein as if set forth in their entirety.

[0071] While the particular process as herein shown and disclosed indetail is fully capable of obtaining the objects and advantages hereinbefore stated, it is to be understood that it is merely illustrative ofthe presently preferred embodiments of the invention and that nolimitations are intended to the details of construction or design hereinshown other than as described in the appended claims.

[0072] For example, one having ordinary skill in the art will realizethat the present invention is also useful in etching anoxide/nitride/oxide (ONO) stack.

What is claimed is:
 1. A method of etching a substrate, comprising stepsof: providing a substrate having oxide over silicon nitride; providing afluorinated plasma comprising an additive fluorocarbon having at leastas many hydrogen atoms as fluorine; and exposing said substrate to saidfluorinated plasma to etch through at least a portion of said oxide toexpose a region of said silicon nitride.
 2. A method according to claim1, further comprising a step of providing said substrate a temperatureof at least 30° C.
 3. A method according to claim 1, wherein saidadditive fluorocarbon comprises at least one of CH₂F₂ and CH₃F.
 4. Amethod according to claim 3, wherein said fluorinated plasma furthercomprises at least one of CHF₃ and CF₄.
 5. A method according to claim4, wherein said fluorinated plasma further comprises argon.
 6. A methodof etching a layered semiconductor substrate, comprising steps of:providing nitride over at least a portion of a semiconductor substrate;forming second different material over at least a portion of saidnitride; and etching through at least a portion of said second materialto expose at least a portion of said nitride; said etching using aplasma comprising an additive fluorocarbon compound having at least asmany hydrogen atoms as fluorine.
 7. A method according to claim 6,wherein said additive fluorocarbon compound comprises at least one ofCH₂F₂ and CH₃F.
 8. A method according to claim 7, wherein said plasmafurther comprises at least one of CF₄ and CHF₃.
 9. A method according toclaim 8, wherein said plasma further comprises argon.
 10. A methodaccording to claim 7, wherein said additive fluorocarbon compound isprovided a gas flow of at least 3% of the total flow for said plasma.11. A method according to claim 10, wherein said additive fluorocarboncompound is provided a gas flow of 3-20% of the total flow for saidplasma.
 12. A method according to claim 11, wherein the total flow forsaid plasma comprises 70-90% of at least one of CHF₃, CF₄ and AR.
 13. Amethod according to claim 12, wherein said total flow comprises at least3% CHF₃.
 14. A method according to claim 13, wherein said total flowcomprises at least 10% CF₄.
 15. A method according to claim 14, whereinsaid total flow comprises at least 33% argon.
 16. A method according toclaim 6, wherein said second material comprises oxide.
 17. A methodaccording to claim 16, wherein said oxide comprises at least one ofundoped silicon oxide and doped silicon oxide.
 18. A method according toclaim 6, further comprising a step of maintaining said substrate at atemperature above 30° C.
 19. A method of plasma processing a layeredstructure, said method comprising the steps of: providing a layeredstructure comprising silicon nitride and silicon oxide over at least aportion thereof; generating a plasma from gases comprising firstfluorocarbons having at least as many hydrogen atoms as fluorine; andemploying said plasma to etch through at least a portion of said siliconoxide and expose a portion of said silicon nitride.
 20. A methodaccording to claim 19, wherein said first fluorocarbons comprise atleast one of CH₂F₂ and CH₃F.
 21. A method according to claim 20, whereinsaid gases further comprise second fluorocarbons comprising at least oneof CHF₃ and CF₄.
 22. A method according to claim 21, wherein said gasesfurther comprise argon.
 23. A method according to claim 22, wherein saidgases comprise about 3-25% of said first fluorocarbons.
 24. A methodaccording to claim 23, wherein said gases comprise about 13-32% of saidsecond fluorocarbons.
 25. A method according to claim 23, wherein saidgases comprise about 3-10% CHF₃.
 26. A method according to claim 25,wherein said gases comprise about 10-22% CF₄
 27. A method according toclaim 26, wherein said gases comprise about 30-60% argon.
 28. A methodaccording to claim 23, wherein said gases comprise about 3-20% CH₂F₂.29. A method according to claim 19, further comprising the steps of:disposing said layered structure upon an electrode of a plasma etchingchamber; and maintaining said electrode at a temperature of at least 30°C. during said etching.